In an injection molding apparatus having a hot half with a hot runner, a movable valve pin can be used to control flow of molding material (e.g., plastic melt) through a nozzle. A portion of the valve pin extends into a melt channel to control the flow of molding material, and a portion of the valve pin extends outside the melt channel for connection to an actuating mechanism.
Typically, a valve pin bushing is installed in a manifold to guide the movement of the valve pin and seal against leakage of molding material out of the melt channel. Sealing generally requires strict tolerances.
Regarding sealing, a hole in the valve pin bushing, through which the pin moves, typically wears over time from the cycling of the valve pin, and eventually, the gap between the valve pin and the hole grows to permit leakage of molding material therethrough. Leakage can be in the form of melt, melt residue, or gasses, any of which can damage the molding apparatus or pollute the environment. Leaking melt and melt residue can also seize the pin in the valve pin bushing, preventing the injection molding apparatus from operating properly. Once the leakage is excessive, the valve pin bushing must be refurbished or replaced along with any other damaged equipment.